A vertebrate lymphoid organ, lying behind the stomach, in which worn-out red blood cells (erythrocytes) are collected and disposed of. It forms part of the peripheral lymphoid system and is a site where antigen-presenting cells stimulate lymphocytes to mount adaptive immune responses against invading pathogens. The spleen consists chiefly of red pulp comprising numerous sinuses where erythrocytes are stored and old ones destroyed. Their remnants are ingested and degraded by macrophages. Interspersed among the red pulp are areas of white pulp. These contain distinct zones of lymphocytes—B cells and T cells—arranged around central arterioles.